a. Field of the Invention
The present invention generally relates to semiconductor manufacturing, and more particularly to focus/aberration monitors used in semiconductor manufacturing.
b. Background of Invention
A conventional deep ultraviolet (DUV) photomask is a transparent plate having a uniform thickness, whereby parts of the transparent plate are covered with non-transmitting (i.e., optically opaque) material in order to create a pattern on a semiconductor wafer when illuminated with, for example, ultra-violet (UV) light. The more recent introduction of extreme ultraviolet (EUV) lithography may require a completely reflective lithography system and thus the non-transmitting material of the photomask is placed on a multi-layer Bragg reflector. However, both DUV and EUV lithography tools that image the photomask patterns onto the semiconductor wafer have various optical components such as optical lenses and/or mirrors that are susceptible to aberrations. Aberrations are generally concerned with the lens state of the lithography tool. For example, focus variations (i.e., defocusing) associated with one or more lenses within the lithography tool may be one type of lens aberration that causes a distortion of the wavefront, which can in turn alter the feature size of a structure being imaged onto the semiconductor wafer surface using the patterned photomask. For instance, the heating of a lens as light passes through it may cause some warpage of the lens and, thus, some defocusing. Additionally, incorrect positioning of the wafer relative to the lens system can produce a defocus error.
Optimally, the semiconductor wafer surface is located at the focal plane of the lens that projects light onto the wafer's surface. However, due to aberration-based changes in the focal plane of the lens relative to the wafer surface, the resolution of the image generated on the wafer's surface varies. As previously discussed, this in turn may cause a change in the feature size and/or location placement of the patterned structures. Thus, aberration monitors may be utilized in order detect such aberrations caused by the lithography tool. Test masks be used in order to monitor aberrations and, therefore, ascertain the performance of the lithography tool.
However, test masks employing phase shifting patterns (e.g., 90° phase shifter) may be used solely for lithography tool evaluation and applied between the imaging photomasks. For example, a phase shifting pattern of a test mask may create a pattern (e.g., parallel lines) on the surface of the semiconductor wafer, whereby based on the geometry of the pattern (e.g., line spacing), a defocusing magnitude for the lithography tool may be determined.
Specifically, once a pattern is imaged by a photomask on the semiconductor wafer surface, the photomask may be removed and replaced by the test mask to evaluate the lithography tool. The test mask may then be removed and replaced by a subsequent patterned photomask used to image a circuit structure on the wafer. This process of using a separate test mask may delay the semiconductor manufacturing process and consequently, among other things, generate additional cost. Moreover, the test masks may be created by etching the phase shifting patterns into the glass substrate of the test mask, which may also contribute to the cost factor.